1. Field of the Invention
The present invention relates to image forming apparatuses using an electrostatic transfer method, such as copy machines, printers or facsimile machines. More particularly, the present invention relates to an image forming apparatus which prevents adhesion of toner or paper powder to the surface of a transfer member for transferring a toner image on a photosensitive member onto a transfer medium such as a transfer paper or an OHP sheet and which thereby prevents the resultant decrease in image quality and contamination of the back of the transfer medium.
2. Discussion of the Background
In image forming apparatuses using an electrostatic transfer method, such as copy machines, printers or facsimile machines, an electrostatic latent image is formed by exposing a photosensitive member (image carrier) previously charged uniformly with optical image information such as reflected light from an original document, and the electrostatic latent image is developed with toner supplied from a developing unit. The resultant toner image is transferred onto a transfer medium by a transfer unit and the transferred image is fixed onto the transfer medium by a fixing unit.
Two types of transfer units are known, a corona transfer type and a contact transfer type. In a corona transfer type transfer unit, a surface of a photosensitive member directly faces a corona charging device, and electrostatic transfer is accomplished by applying a charge of an inverted polarity to that of a toner image carried on the photosensitive member, through corona discharge, to a transfer medium passing through the gap between the corona charging unit and the photosensitive member. In a contact transfer type transfer unit, a bias roller serving as a transfer device faces a photosensitive member via an endless transfer belt made of a material having a certain electric resistance, and a toner image is transferred from the photosensitive member onto a transfer medium by applying a transfer bias to the bias roller when the transfer medium carried by the transfer belt is transported to a transfer position. The transfer belt having a certain electrical resistance has a surface coat layer having a prescribed elongation percentage (for example, a layer coated by a coating material using fluorine) formed on an elastic substrate made of, for example, EPDM and chloroprene or singly chloroprene.
A contact transfer type transfer unit in which a transfer belt is used and a bias is applied onto the transfer belt has advantages of a small amount of generated ozone, a small capacity of power supply, and a satisfactory separating and transporting performance of the transfer medium.
However, the contact transfer type transfer unit using a transfer belt has a problem in the cleaning performance of the transfer belt. More specifically, residual toner remaining on a surface of the photosensitive member or toner floating within the apparatus tends to easily adhere onto the transfer belt electrostatically or under a contact pressure, and the toner adhering onto the transfer belt causes back contamination of a transfer medium transported on the transfer belt. Further, in the contact transfer type transfer unit, the heat of the photosensitive member is transferred to the transfer belt due to the contact between the transfer belt and the photosensitive member and thereby the surface temperature of the belt is increased. As a result, the toner firmly adheres to the transfer belt, making it difficult to remove. This phenomenon is known as toner adhesion.
Such toner adhesion to the transfer belt causes defective separation of a transfer medium from the transfer belt, resulting in jamming of the transfer medium and wear of the cleaning blade edge. Further, the toner adhering to the transfer belt moves back onto the surface of the photosensitive member and causes a decrease in the quality of the transferred image. Furthermore, in a machine mounting a charging roller, the toner having moved from the transfer belt to the photosensitive member adheres to the charging roller, which may cause unstable charging potential or an unstable image forming process. If toner penetrates into fine cracks of the transfer belt, the toner penetrated into the fine cracks cannot be sufficiently cleaned with a cleaning device for the transfer belt, resulting in accumulation of the toner in the cracks of the transfer belt. As a result, defective separation of a transfer medium from the transfer belt and resulting jamming of the transfer medium, and wear of the cleaning blade edge may be caused.
The transfer belt may be further contaminated by paper powder produced from the transfer medium. Paper powder adheres to the transfer belt, penetrates into the fine cracks on the belt surface and accumulates there, and so is difficult to remove with the cleaning device for the transfer belt. Such accumulation of paper powder may cause accumulation or adhesion of toner around the paper powder. Further, entanglement and adhesion of paper powder at the edge of the cleaning blade tends to decrease cleaning performance of the cleaning device, resulting in leaving residue of toner on the transfer belt.
In an apparatus using recycled toner, because of the properties of the recycled toner, it is difficult to remove the recycled toner adhering to a transfer belt, resulting in a very poor cleaning performance of the transfer belt.
Toner or paper powder adhering to a transfer belt is generally removed with a cleaning device such as a cleaning blade rubbing the surface of the transfer belt. Various methods are proposed for improving the cleaning performance of a cleaning device or preventing back contamination of a transfer medium.
For example, Japanese Laid-open Patent Publication No. 8-278707 proposes an image forming apparatus having a lubricant feeding member feeding a lubricant onto the surface of a transfer member for improving the cleaning performance of the transfer member by reducing frictional coefficient of the surface of the transfer member, and an image forming apparatus having a lubricant feed device feeding a lubricant onto a photosensitive member so that the lubricant fed onto the photosensitive member is coated onto the surface of the transfer member.
Japanese Laid-open Patent Publication No. 8-185060 proposes a transfer/transport belt apparatus, in which a foaming film layer is formed on the surface of a transfer belt so that toner is collected inthe foam cells of the foaming film layer, thus preventing back contamination of a transfer medium.
Japanese Laid-open Patent Publication No. 8-305181 proposes a transfer/transport apparatus in which the thickness of a surface coat layer of a transfer belt is made smaller than the toner particle diameter for preventing defective cleaning of the surface of the transfer belt so that, even when cracks are produced in the surface coat layer of the transfer belt, toner particles do not penetrate into the cracks and can easily be scraped off with a cleaning device (cleaning blade).
However, when coating a lubricant onto the surface of a transfer member, as is disclosed in the aforementioned Japanese Laid-open Patent Publication No. 8-278707, coating blurs of the lubricant tend to occur, and cleaning blurs may be caused by coating blurs of the lubricant. Further, even when the aforementioned lubricant is coated, if cracks occur in the transfer member, a satisfactory cleaning performance may not be made.
When a foamed film layer is formed on the surface of a transfer belt as is disclosed in the aforementioned Japanese Laid-open Patent Publication No. 8-185060, toner adhesion may be caused by accumulation of toner in the foam cells of the foamed film layer.
Further, even when the thickness of a surface coat layer of a transfer belt is made smaller than the particle diameter of toner, as is disclosed in Japanese Laid-open Patent Publication No. 8-305181, toner may be accumulated in the cracks of the transfer belt, and so toner adhesion may occur.
In an image forming apparatus in which a contact transfer type transfer device using a transfer belt is used, therefore, it is very important to improve the cleaning performance of an endless transfer member such as a transfer belt. More specifically, it is desired that the performance of removing toner and paper powder adhering onto the endless transfer member is improved, so that back contamination of a transfer medium is reduced.